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<H2><A NAME="definition-computer-network"></A> <A NAME="s2">2.</A> <A HREF="networking-concepts-HOWTO.html#toc2">What is a `computer network'?</A></H2>

<P>
<A NAME="definition-node"></A> 
A computer network is just a set of stuff for nodes to talk to each
other (by `nodes' I mean computers, printers, Coke machines and
whatever else you want).  It doesn't really matter <B>how</B> they
are connected: they could use fiber-optic cables or carrier pigeons.
Obviously, some choices are better than others (especially if you have
a cat).</P>
<P>
<A NAME="definition-sub-network"></A> </P>
<P>Usually if you just connect two computers together, it's not called
a network; you really need three or more to become a network.  This is
a bit like the word `group': two people is just a couple of guys, but
three can be an `group'.  Also, networks are often hooked together, to
make bigger networks; each little network (usually called a
`sub-network') can be part of a larger network.</P>
<P>
<A NAME="definition-network-link"></A> </P>
<P>The actual connection between two computers is often called a
`network link'.  If there's a bit of cable running out of the back of
your machine to the other machines, that's your network link.</P>

<P>There are four things which we usually care about when we talk
about a computer network:</P>
<P>
<DL>
<DT><B>Size</B><DD><P>
<A NAME="definition-LAN"></A> 
If you simply connect your four computers at home together,
you have what is called a LAN (Local Area Network).  If
everything is within walking distance, it's usually called a
LAN, however many machines are connected to it, and whatever
you've built the network out of.</P>
<P>
<A NAME="definition-WAN"></A> </P>
<P>The other end of the spectrum is a WAN (Wide Area Network).
If you have one computer in Lahore, Pakistan, one in Birmingham,
UK and one in Santiago, Chile, and you manage to connect them,
it's a WAN.</P>

<DT><B>Topology: The Shape</B><DD><P>
<A NAME="definition-topology"></A> 
Draw a map of the network: lines are the</P>
<P>
<A HREF="#definition-network-link">network links</A>, and
each node is a dot.  Maybe each line leads into a central node
like a big star, meaning that everyone talks through one
point (a 
<A NAME="definition-star-topology"></A>  `star topology'):
<BLOCKQUOTE><CODE>
<PRE>
    o   o   o
     \_ | _/
       \|/
  o-----o-----o
      _/|\_
     /  |  \
    o   o   o
</PRE>
</CODE></BLOCKQUOTE>
</P>

<P>Maybe everyone talks in a line, like so:
<BLOCKQUOTE><CODE>
<PRE>
    o------o------o-------o--------o
    |                              |
    |                              |
    |                              o
    |                              |
    o                              |
                                   o
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>Or maybe you have three subnetworks connected through one node:
<BLOCKQUOTE><CODE>
<PRE>
                o
    o           |  o--o--o
    |           |  |
    o--o--o--o--o  o
           \       |
            o------o
           /       |
    o--o--o--o--o  o
    |           |  |
    o           |  o--o
                o
</PRE>
</CODE></BLOCKQUOTE>
</P>

<P>You'll see many topologies like these in real life, and
many far more complicated.</P>

<DT><B>Physical: What It's Made Of</B><DD><P>
<A NAME="definition-sneakernet"></A>  The second thing to care
about is what you've built the network out of.  The cheapest
is `sneakernet', where badly-dressed people carry floppy disks
from one machine to the others.  Sneakernet is almost always a
<A HREF="#definition-LAN">LAN</A>.  Floppies cost less than
$1, and a solid pair of sneakers can be got for around $20.</P>

<P>
<A NAME="definition-modem"></A>  The most common system used to connect home networks
to far bigger networks is called a
`modem' (for MODulator/DEModulator), which turns a normal
phone connection into a network link.  It turns the stuff the
computer sends into sounds, and listens to sounds coming from
the other end to turn them back into stuff for the computer.
As you can imagine, this isn't very efficient, and phone lines
weren't designed for this use, but it's popular because phone
lines are so common and cheap: modems sell for less than $50,
and phone lines usually cost a couple of hundred dollars a
year.</P>

<P>
<A NAME="definition-ethernet"></A> 
<A NAME="definition-UTP"></A> 
<A NAME="definition-10base2"></A> 
<A NAME="definition-10baseT"></A> 
<A NAME="definition-100baseT"></A> 
<A NAME="definition-coax"></A> 
<A NAME="definition-gigabit-ethernet"></A> 
<A NAME="definition-terminator"></A> 
<A NAME="definition-hub"></A> 
The most common way to connect machines into a LAN is to use
Ethernet.  Ethernet comes in these main flavors (listed from
oldest to newest): Thinwire/Coax/10base2, UTP (Unshielded
Twisted Pair)/10baseT and UTP/100baseT.  Gigabit ethernet (the
name 1000baseT is starting to get silly) is starting to be
deployed, too.  10base2 wire is usually black coaxial cable,
with twist-on T-pieces to connect them to things: everyone
gets connected in a big line, with special `terminator' pieces
on the two ends.  UTP is usually blue wire, with clear
`click-in' phone-style connectors which plug into sockets to
connect: each wire connects one node to a central `hub'.  The
cable is a couple of dollars a meter, and the 10baseT/10base2
cards (many cards have plugs for both) are hard to get brand new.
100baseT cards, which can also speak 10baseT as well, are ten
times faster, and about $30.</P>

<P>
<A NAME="definition-fiber"></A>  On the other end of the
spectrum is Fiber; a continuous tiny glass filament wrapped in
protective coating which can be used to run between
continents.  Generally, fiber costs thousands.</P>

<P>
<A NAME="definition-network-interface"></A> We usually call
each connection to a node a `network interface', or
`interface' for short.  Linux gives these names like `eth0'
for the first ethernet interface, and `fddi0' for the first
fiber interface.  The `/sbin/ifconfig' command lists them.</P>

<DT><B>Protocol: What It's Speaking</B><DD><P>
<A NAME="definition-protocol"></A> 
The final thing to care about is the language the two are
speaking.  When two 
<A HREF="#definition-modem">modems</A>
are talking to each other down a phone line, they need to
agree what the different sounds mean, otherwise it simply
won't work.  This convention is called a `protocol'.  As
people discovered new ways of encoding what the computer says
into smaller sounds, new protocols were invented; there are at
least a dozen different modem protocols, and most modems will
try a number of them until they find one the other end
understands.</P>

<P>Another example is the 
<A HREF="#definition-100baseT">100baseT</A> network mentioned above: it uses the same
physical 
<A HREF="#definition-network-link">network links</A>
(
<A HREF="#definition-UTP">UTP</A>) as
<A HREF="#definition-10baseT">10baseT</A> above, but talks
ten times as fast.</P>
<P>
<A NAME="definition-link-level-protocol"></A> 
<A NAME="definition-hop"></A> </P>
<P>These two protocols are what are called `link-level'
protocols; how stuff is handed over the individual network
links, or `one hop'.  The word `protocol' also refers to other
conventions which are followed, as we will see next.</P>
</DL>
</P>

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